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To describe the essence of Tyrannosaurus rex, the most terrifying predator that ever lived, University of Chicago paleontologist Paul Sereno offers this: "Jaws on fast-running legs." The monster had enormous jaws, which it used to grab and crunch into its prey and which largely explain why it's head was so huge. T. rex's legs were massive as well, allowing the 2.5-ton dinosaur to run its victims down like a racehorse.

For years, scientists assumed those distinguishing features, along with T. rex's relatively puny arms, had evolved as the creature itself got bigger. "When a building gets bigger," says Stephen Brusatte, a former graduate student of Sereno's and now at the American Museum of Natural History, "you have to build it differently. That's what we thought happened with T. rex."

Now they know better. Sereno, Brusatte and a team of American and Chinese scientists  plus a Massachusetts ophthalmologist  have just announced the discovery of a 125 millionyearold animal that had the same distinctive build as T. rex, but at only about 1/100 the weight. "It's all there," says Sereno, "including the dinky arms." But the new dinosaur, named Raptorex kriegsteini and described in the online journal Science Express, would have been a bit taller and about as heavy as an adult human. Says Brusatte: "It really throws a wrench into the story."

The way Raptorex came to light, meanwhile, is a story in itself. About three years ago, Sereno, who usually does his own fossil-hunting, was approached by ophthalmologist and avid fossil collector Henry Kriegstein for help in identifying a newly purchased specimen. Kriegstein had bought the fossil legally, but when Sereno saw it, he became convinced it was from China and that it had almost certainly been smuggled out of that country. "I told him that I'd help," says Sereno, "but only if he was willing to donate the fossil to science and let us return it to China when we were done studying it."

Kriegstein agreed. Sereno, in turn, gave the remarkable specimen Kriegstein's surname (in honor of his parents, Holocaust survivors who are still alive), and listed Kriegstein as a co-author on perhaps the most important paleontology paper of the year. "In the normal course of things," says Sereno, "this fossil could have ended up on someone's mantelpiece or been forgotten in an attic somewhere and lost to science. Now China gets its property back, and Dr. Kriegstein has found immortality for his family. Everybody wins."

That includes the scientific community. Paleontologists aren't just interested in what dinosaurs looked and acted like; they also want to know how they fit into their environment. From what scientists already know about the ancient lake beds where Raptorex was originally found, for example, they know it had some stiff competition. "They would have co-existed with velociraptor-like dinosaurs," says Sereno  the human-scale carnivores that starred in Jurassic Park. But they would have hunted very differently: velociraptors, Sereno explains, "had long, grasping arms with clawed hands." They also had a large, sickle-shaped claw on their middle toes, probably used for slashing prey. It was most likely only after the prey was dead that their mouths got into the act.

Raptorex's mouth, by contrast, was most likely a primary weapon  like that of its distant descendant Tyrannosaurus. But that alone wasn't enough of an advantage to let it evolve into the predator that would ultimately dominate North America and Asia. That probably didn't happen until larger predatory dinosaurs went extinct for other reasons, say the scientists, allowing Raptorex-like creatures to begin growing. Once they started to get into the league of the big predators, though, where speed and bone-crushing jaw strength would let them range farther and crunch the bones of the biggest prey, there was no competition at all. By about 90 million years ago at the latest, T. rex  or as we might now say, the king-size version of Raptorex  was unchallenged. "There was no turning back," says Sereno, referring to the leading theory on why the dinosaurs became extinct, "until the comet hit."